Oxidization of hydrocarbon compounds with molecular oxygen, and particularly oxidation with air have been examined for many years, and many methods have been disclosed. In the autoxidation of hydrocarbon compounds, the oxidation of cyclohexane is particularly important from an industrial perspective. The obtained cyclohexanone and cyclohexanol are very important compounds as raw materials for nylon 6 and nylon 6,6.
The oxidation of hydrocarbon compounds with molecular oxygen progresses via the corresponding hydroperoxide. The selectivity of an oxidation reaction from a hydrocarbon compound to a hydroperoxide is high, but when hydroperoxides decompose, various byproducts are generated, and therefore the selectivity decreases. In order to prevent this decrease in selectivity, a method is generally adopted in which the oxidation reaction is ended at a stage in which the conversion is low, and the hydroperoxides in the reaction solution that remain without decomposing are decomposed at the next step to produce a ketone and/or alcohol.
Methods for decomposing hydroperoxides can be broadly categorized into two types of methods. One is a method in which a small amount of a transition metal compound or the like is added to decompose the hydroperoxide, and the other is a method in which a hydroperoxide is contacted with an alkaline aqueous solution to decompose the hydroperoxide. Among these, with the former type of method, the rate of decomposition of the hydroperoxide is slow, and the decomposition selectivity is also not very high, and therefore ordinarily the latter type of method is adopted. Note that with the process of the latter type of method, the neutralization of carboxylic acid produced as a byproduct by the oxidation reaction, and the hydrolysis of esters produced as byproducts are simultaneously performed, and therefore this is known as a saponification step.
In the saponification step, in order to decompose a hydroperoxide with high selectivity, it is crucial that hydroperoxide is moved rapidly to the alkaline water phase. This is because radical decomposition of hydroperoxides having low selectivity progresses in the oil phase, and the decomposition selectivity decreases. With the saponification step, in order that hydroperoxides are dissociated and rapidly moved to the water phase, a strong alkaline aqueous solution such as alkali metal hydroxides or the like is used. However, the carboxylic acids generated in the oxidizing step reduce the pH of the saponification step. In order to prevent this, a method is widely adopted in which the saponification step is divided into two steps: a carboxylic acid neutralization step and a hydroperoxide decomposition step, and the oxidation reaction solution and the alkaline aqueous solution are brought into counter-flow-contact. This is called a two-step saponification process. However, the alkaline water phase that is discharged from the carboxylic acid neutralization step has a low pH value, and therefore this cannot be recirculated and used. It is also difficult to recycle and reuse the strong alkali. That is, the two-step saponification process is excellent in hydroperoxide decomposition selectivity, but it is a process with high alkali consumption.